Apparatus for electrostatically forming a developer image on a conveyed recording medium including an array of control electrodes each having an optimized pitch

ABSTRACT

The array pitch `d` between the control electrode strips with respect to the conveyed direction of the recording medium is determined based on the order of applying operating voltage, the resolution `x` of the image with respect to the conveyed direction, and the number `m` of the control electrode strips. Illustratively, in order to obtain a particular resolution `x` with respect to the conveyed direction of the paper, by taking into consideration the reduction in dot interval or conveyed distance by the conveyance of the recording paper, the array pitch `d` between the control electrode strips with respect to the conveyed direction of the recording medium is set as the dot distance produced when the paper is not moved, plus the above reduction. This can be written as d=(n+1/m)/x where n is a natural number.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to an image forming apparatus such as adigital copier, facsimile machine, digital printer, plotter etc., andmore particularly relates to an image forming apparatus in which animage is formed on a recording medium by causing the developer to jumpthereto.

(2) Description of the Prior Art

There have been known image forming apparatuses which, in accordancewith an image signal, form a visual image on a recording medium such aspaper etc. Japanese Patent Application Laid-Open Hei 7 No. 25,057, forexample, discloses an image forming apparatus wherein a toner image isformed, in accordance with an image signal, on a recording medium bycausing the toner as the developer to jump whilst the voltage beingapplied to a control electrode which is placed in the jumping path ofthe toner is varied.

More specifically, the control electrode of this apparatus, has a numberof gates to be the passage holes for the toner, which are arrangedgrid-wise, i.e., at regular intervals in the conveyed direction of therecording paper, creating a predetermined resolution, arranged in a 4gate unit slant-wise in the direction perpendicular to the conveyeddirection, forming a coarse pitch. In this arrangement, the potential ofthe gates is controlled in a regular cycle in a time-divisional mannerin accordance with the image signal, so as to control toner transfer, toform a toner image on the recording paper.

However, in accordance with the above conventional apparatus, when thepotential of the gates is controlled in a time-divisional manner withrespect to the perpendicular direction to the conveyed direction of thepaper (to be referred to hereinbelow as the row-direction) in order toattain the predetermined resolution with respect to the conveyeddirection of the paper, it is necessary to take into consideration therelationship between the pitch between the gates with respect to theconveyed direction of the paper and the conveying speed of the paper tobe recorded, and this has complicated the driver control of the gates.

Besides, if the relationship between the pitch of the gates with respectto the conveyed direction of the recording paper and the conveying speedof the paper is not correct, the control signal becomes inactive, asshown in FIG. 1, from the time when the time-divisional switchingcontrol of control electrodes S1-S4 for one line has been completedbased on the line synchronizing signal to the time when the nexttime-divisional switching control starts, thus resulting in reduction ofthe effective time available for printing.

SUMMARY OF THE INVENTION

The present invention has been devised in view of the above problems,and it is therefore an object of the present invention to provide animage forming apparatus which, though still has a simple structure, canperform a high speed image forming operation without having any inactivetime between successive lines being printed.

In order to attain the above object, the present invention isconfigurated as follows:

In accordance with the first aspect of the invention, an image formingapparatus wherein an image is formed on a recording medium with thedeveloper by performing voltage application in a time-divisional manner,to a plurality of control electrode strips having passage holes thatallow the charged developer particles to pass therethrough, ischaracterized in that the array pitch of the control electrode stripswith respect to the conveyed direction of the recording medium isdetermined based on the order of performing voltage application, theresolution of the image with respect to the conveyed direction and thenumber of the control electrode strips.

In accordance with the second aspect of the invention, an image formingapparatus wherein an image is formed on a recording medium with thedeveloper by performing voltage application in a time-divisional manner,to a plurality of control electrode strips having passage holes thatallow the charged developer particles to pass therethrough, ischaracterized in that when voltage application is performed in the samedirection as the arranged order of the control electrode strips withrespect to the conveyed direction of the recording medium, the arraypitch `d` of the control electrode strips with respect to the conveyeddirection of the recording medium is set so as to satisfy the followingrelation:

    d=(n+1/m)/x

(where n is a natural number)

where x is the resolution of the image and m is the number of thecontrol electrode strips.

In accordance with the third aspect of the invention, an image formingapparatus wherein an image is formed on a recording medium with thedeveloper by performing voltage application in a time-divisional manner,to a plurality of control electrode strips having passage holes thatallow the charged developer particles to pass therethrough, ischaracterized in that when voltage application is performed in thereverse direction to the arranged order of the control electrode stripswith respect to the conveyed direction of the recording medium, thearray pitch `d` of the control electrode strips with respect to theconveyed direction of the recording medium is set so as to satisfy thefollowing relation:

    d=(n-1/m)/x

(where n is a natural number)

where x is the resolution of the image and m is the number of thecontrol electrode strips.

Next, operation of the present invention is described hereinbelow.

In accordance with the image forming apparatus of the first feature ofthe invention, the conveyed distance of the paper while one controlelectrode strip is activated, is determined based on the resolution andthe number of the control electrode strips. Here, the distance betweenthe image dots formed by neighboring control electrode strips is reducedby the above conveyed distance compared to the dot distance (i.e. thedistance between the neighboring gates arranged in the conveyeddirection) formed when the paper is not moved. Taking into considerationthis fact, the array pitch of the gates in the control electrode stripswith respect to the conveyed direction of the recording medium is basedon the order or direction of voltage application, the resolution of theimage in the conveyed direction of the paper (the dot density in theimage) and the number of the control electrode strips.

In accordance with the image forming apparatus of the second feature ofthe invention, the conveyed distance `z` of the paper while one controlelectrode strip is activated, is determined as the reciprocal of theproduct of the resolution `x` and the number of control electrode strips`m`, i.e., (=1/(mx)). Here, in the case where voltage is applied to thecontrol electrode strips in the same order as the arranged order of thecontrol electrode strips with respect to the conveyed direction of thepaper, the distance between the image dots formed by neighboring controlelectrode strips is reduced by the above conveyed distance `z` comparedto the dot distance `y` (=n/x) formed when the paper is not moved.Accordingly, in order to obtain a particular resolution `x` with respectto the conveyed direction of the paper, the array pitch `d` betweencontrol electrode strips with respect to the conveyed direction of therecording medium should and may be set as the distance `y` plus theconveyed distance `z`, by taking into consideration the reduction of theconveyed distance `z`. This can be written as d=(n+1/m)/x where n is anatural number.

In accordance with the image forming apparatus of the third feature ofthe invention, the conveyed distance `z` of the paper while one controlelectrode strip is activated, is determined as the reciprocal of theproduct of the resolution `x` and the number of control electrode strips`m`, i.e., (=1/(mx)). Here, in the case where voltage application isperformed in the reverse order to that of the arrangement of the controlelectrode strips with respect to the conveyed direction of the paper,the distance between the image dots formed by neighboring controlelectrode strips is increased by the above conveyed distance `z`compared to the dot distance `y` (=n/x) formed when the paper is notmoved. Accordingly, in order to obtain a particular resolution `x` withrespect to the conveyed direction of the paper, the array pitch `d`between control electrode strips with respect to the conveyed directionof the recording medium should and may be set as the distance `y` minusthe conveyed distance `z`, by taking into consideration the reduction ofthe conveyed distance `z`. This can be written as d=(n-1/m)/x where n isa natural number.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram for explaining the operation of a conventional imageforming apparatus;

FIG. 2 is a diagram illustrating the configuration of an image formingapparatus in accordance with the embodiment of the invention;

FIG. 3 is a diagram for explaining the configuration of the controlelectrode unit constituting an image forming apparatus in accordancewith the embodiment of the invention; and

FIG. 4 is a timing chart for explaining the driver controlling method ofthe control electrode unit constituting an image forming apparatus inaccordance with the embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiment of the invention will hereinafter be described withreference to FIGS. 2 through 4. In these figures, common elements orcorresponding elements will be allotted with the same referencenumerals, so that the description of such elements will not be repeated.In the following embodiment, description will be made of an examplewhere an image forming apparatus for the embodiment has a configurationfor negatively charged toner.

An image forming apparatus in accordance with this embodiment iscomposed of, in view of functions, a paper feeding unit 101, printingunit 102 and fixing unit 103, as shown in FIG. 2. Hereinbelow, each partwill be described illustratively.

First, paper feeding unit 101 is arranged on the input side of printingunit 102 which is located at the approximately center of the apparatus.This paper feeding unit 101 is composed of a paper tray 1 for storingpaper 2 as the recording medium, a pickup roller 3, a paper feed roller4 and a paper guide roller 5 and a paper feed sensor 15.

Here, pickup roller 3 is rotationally driven by an unillustrated drivermeans, to thereby pick up only a single sheet of recording paper 2 frompaper tray 1 and deliver it to paper feed roller 4. The thus fed paper 2is further conveyed to printing unit 102 (to be described hereinbelow)by means of paper feed roller 4 arranged below the guide of paper guideroller 5. Paper feed sensor 15 detects the conveyance state of recordingpaper 2.

Fixing unit 103 disposed on the output side of printing unit 102, iscomposed of a heat roller 20, a heater 18, a pressing roller 19, atemperature sensor 21, a paper discharge sensor 25 and a paper dischargeroller 22. Here, heat roller 20 and pressing roller 19 are rotationallydriven by an unillustrated driver, whereby the toner image formed onpaper 2 at the aftermentioned printing unit 102 is heated and pressed soas to be fixed thereon.

In the above fixing unit, heat roller 20 is made up of an aluminum pipeof 2 mm thick, for example. Heater 18 is of a halogen lamp for instance,and incorporated in heat roller 20. Pressing roller 19 is made ofsilicone for example, and is provided opposite heat roller 20.

Heat roller 20 and pressing roller 19 which are arranged opposite toeach other, are pressed against one another, holding paper 2 in betweenand pressing it, with a pressing load, e.g. 2 kg, from unillustratedsprings etc., provided at both ends of their shafts. Paper 2 with atoner image formed thereon is pressed between, whilst being heated by,heat roller 20 and pressing roller 19, so that the toner image will befixed to paper 2.

Temperature sensor 21 measures the surface temperature of heat roller20. The measurement result is controlled by an unillustrated controllerso as to govern the on/off operation of heater 18, thus maintaining thesurface temperature of heater roller 20 at a fixed temperature, e.g.,150° C. Paper discharge sensor 25 detects the passage of recording paper2 between pressing roller 19 and heat roller 20.

It should be noted that the materials of heat roller 20, heater 18,pressing roller 19, etc., as well as the surface temperature of heatroller 20 and the like, are not specifically limited to the abovereferences. Further, fixing unit 103 may have a fixing configuration inwhich the toner image is either only heated or pressed to affix itselfto recording paper 2.

Next, detailed description will be made of printing unit 102, whichdirectly form a toner image on the paper by causing the toner as thedeveloper to jump by electric force whilst controlling the jumpingpassage of the toner in accordance with the image signal.

Illustratively, printing unit 102 has, on the underside of the conveyingpassage of paper 2, a toner storage tank 12 for storing toner 24 as thedeveloper, a drum-shaped toner support (sleeve) 11 for supporting toner24 on its outer peripheral surface and a doctor blade 13 whichnegatively electrifies toner 24 and regulates the thickness of the tonerlayer carried on the peripheral surface of toner support 11.

Toner support 11 is electrically grounded and is configured so that itcan carry toner 24 in the form of a layer on its peripheral surface. Thetoner support is driven by an unillustrated driver means in a directionindicated by arrow A in the figure, with its surface speed set at 100mm/sec, for example. Doctor blade 13 is arranged on the upstream sidewith respect to the rotational direction of toner support 11, spaced ata distance of 60 μm, for example, from the outer peripheral surface oftoner support 11. The thickness of the toner layer is regulated by thisgap.

Toner 24 is of a non-magnetic type having a mean particle diameter of,for example, 6 μm and is tribo-electrified with static charge of -5 to-10 (μC/g) by doctor blade 13. Here, the distance between doctor blade13 and toner support 11 as well as the mean particle size, the amount ofstatic charge, etc., of toner 24 are not particularly limited to theabove-mentioned values.

Provided on the upper side of the conveying path of paper 2 is a papersuction fan 6 which is housed by a cover (not designated by a referencenumeral) that opens toward paper 2. A rearside electrode 7 is providedinside the cover so that it can come in contact with paper 2, facing theperipheral surface of toner support 11.

Arranged on the lower side of recording paper 2 is a control electrodeunit 8 located between toner support 11 and rearside electrode 7. Thiscontrol electrode unit 8 is provided with gates which, as will bedetailed, control toner transfer, that is, function so as to allow ordisallow the passage of the toner jumping from toner support 11 torearside electrode 7.

Rearside electrode 7 has a fixed potential of e.g., 2 kV, supplied by arearside electrode driver power source 14, thus generating an electricfield for causing toner 24 to jump from toner support 11. Paper suctionfan 6 exhausts air from the cover interior so as to generates a negativepressure, thus attracting recording paper 2 to rearside electrode 7.Control electrode unit 8, as shown in FIG. 3, has a insulative board(not shown), a plurality of row-control electrode strips 8R (8R-1 to8R-8) arranged in parallel to one another having a pitch of d, on oneside of the insulative board, perpendicularly to conveyed direction B ofpaper 2. The control electrode further has column-control electrodestrips 8C (8C-1 to 8C-640) arranged on the opposite side of theinsulative board at intervals of a predetermined pitch, in such a mannerthat they cross over row-control electrode strips 8R.

Formed at each of the intersections between these row-control electrodesstrips 8R and column-control electrode strips 8C is a hole 23 formingthe passage of the toner (to be referred to hereinbelow as `gate`).Control electrode unit 8 used here is composed of gates arranged in arectangular array of 8 rows and 640 columns. The arrangement of gatesshould not be limited to this, the numbers of gates in the directions ofthe rows and columns may be of any number.

Here, each column-control electrode strip 8C is arranged at apredetermined angle with row-control electrode strips 8R so that thegates on the same column-control electrode strip 8C are positioned atintervals of a pitch `a` with respect to the width of paper 2. Further,all the column-control electrode strips 8C are arranged so that thegates are positioned keeping the pitch `a` across the width of recordingpaper 2.

This pitch `a` determines the resolution of the image formed onrecording paper 2, with respect to the width thereof. For example, inorder to obtain a resolution of 600 dpi (dot per inch), 600 gates arearranged for one inch width; in this case, pitch `a` will be 1/600 inch.Since the gates are formed at the intersections between row-controlelectrode strips 8R and column-control electrode strips 8C, the totalnumber of the gates in a 8 row×640 column configuration is 5,120.

Column-control electrode strips 8C and row-control electrode strips 8Rare selectively applied with a voltage, either 0V or 300 V, for example,from a column-control electrode driver circuit 17 and a row-controlelectrode driver circuit 16 shown in FIG. 2, respectively.Column-control electrode driver circuit 17 and row-control electrodedriver circuit 16 are controlled by an unillustrated controller so as tooutput either 0 V or 300 V to column-control electrode strips 8C androw-control electrode strips 8R, in accordance with the image signal. Inthe case of this embodiment, only the gates located at interactionsbetween column-control electrode strips 8C and row-control electrodestrips 8R, both having a voltage of 300 V applied thereto will allowpassage of the toner.

Next, the method of operation in control electrode unit 8 will bedescribed by exemplifying a case where row-control electrode strips 8Rare operated or scanned in the same order (voltage is applied to theelectrode strips in the same order) as the arranged order of row-controlelectrode strips 8R with respect to the conveyed direction of paper 2.

As shown in FIG. 4, row-control electrode strips 8R are operated basedon a clock CLK signal producing regularly occurring pulses, in atime-divisional manner, sequentially starting from row-control electrodestrip 8R-1 to row-control electrode strip 8R-8. It should be noted thattwo or more of row-control electrode strips 8R are never operated at thesame time. On the other hand, each of column-control electrode 8C isoperated by a signal having a suitable pattern that changes with time inaccordance with the position of the row-control electrode strip 8R beingoperated, thus forming the image.

In order to exactly obtain a predetermined resolution with respect tothe conveyed direction B of paper 2, the array pitch `d` betweenneighboring row-control electrode strips 8R shown in FIG. 3, needs to beset in relation with the timing of operation. More specifically, whenthe resolution in the conveyed direction B of recording paper 2 is 600dpi, the array pitch `d` is set so that paper 2 will travel by only1/600 inch during the time span of one line, i.e., one cycle ofoperation from row-control electrode strip 8R-1 to row-control electrodestrip 8R-8.

The method of determining the array pitch `d` between row-controlelectrode strips to achieve a predetermined resolution will beillustrated further. In this embodiment, the number `m` of row-controlelectrode strips 8R is 8, and therefore, the conveyed distance `z` ofrecording paper 2 while a single row-control electrode strip 8R isoperated, will be 1/4800 inch (the reciprocal of the product of theresolution `x` and the number of row-control electrode strips `m`). Inthis case, since row-control electrode strips 8R are adapted to beoperated or scanned in the same order as the arranged order ofrow-control electrode strips 8R with respect to the conveyed direction Bof recording paper 2, the distance between the toner image dots formedby the respective control electrode strips is shorter by the aboveconveyed distance `z` of the paper than the distance `y` (=n/x) which isdefined to be the distance between the toner image dots formed whenrecording paper 2 is not moved.

Accordingly, in order to obtain a particular resolution `x` with respectto the conveyed direction of paper 2, the array pitch `d` betweenrow-control electrode strips 8R with respect to conveyed direction B ofrecording medium 2 should be set as the distance `y` plus the conveyeddistance `z`, by taking into consideration the reduction of the conveyeddistance `z` in the pitch of the toner image by the conveyance of paper2. In this case, d=(n+1/m)/x=n/600+1/4800 (inch), where n is a naturalnumber.

By setting the array pitch `d` of row-control electrode strips 8R asabove, the resolution of the image formed on paper 2 when row-controlelectrode strips 8R are operated continuously with a fixed cycle asshown in FIG. 4, can exactly be set to 600 dpi with respect to theconveyed direction B. In other words, when row-control electrode strips8R are arranged having an array pitch `d` as stated above, it ispossible to continuously effect time-divisional operation of row-controlelectrode strips 8R, whereby it is possible to eliminate the necessityfor providing inactive time during image forming.

In the above description of this embodiment, an example was shown inwhich row-control electrode strips 8R are operated or scanned in thesame order as the arranged order of row-control electrode strips 8R withrespect to the conveyed direction B of paper 2. On the other hand, ifrow-control electrode strips 8R are operated or scanned in the reverseorder, the pitch of the toner image dots formed by row-control electrodestrips 8R will become greater by the above conveyed distance `z` thanthe distance `y` (=n/x) between the toner image dots formed when paper 2is not moved.

Accordingly, in order to obtain a particular resolution `x` with respectto the conveyed direction B of paper 2, the array pitch `d` betweenrow-control electrode strips 8R with respect to conveyed direction B ofrecording medium 2 should be set as the distance `y` minus the conveyeddistance `z`, by taking into consideration the increase of the conveyeddistance `z` in the pitch of the toner image by the conveyance of paper2. In this case, d=(n-1/m)/x=n/600-1/4800 (inch).

As has been detailed heretofore, in the apparatus of this embodiment,the array pitch `d` of row-control electrode strips 8R with respect tothe conveyed direction B of recording medium 2 is determined based onthe order of applying the operating voltage to row-control electrodestrips 8R, resolution `x` in the toner image with respect to theconveyed direction of paper 2, and the number `m` of the row-controlelectrode strips. Therefore, it is possible to operate the row-controlelectrode strips 8R based on a clock CLK signal having a fixed cycle, tothereby provide an image having a predetermined resolution `x`.

As has been apparent from the description, in accordance with theinvention, the array pitch in the control electrode (row-controlelectrode strips 8R) with respect to the conveyed direction of therecording medium is adapted to be determined based on the order ofapplying of the operating voltage, the resolution in the image withrespect to the conveyed direction of the recording medium, and thenumber of control electrode strips (row-control electrode strips 8R). Asa result, it becomes unnecessary to provide invalid time during imageforming, thus making it possible to efficiently reduce the time requiredfor image forming.

Further, as a result of not providing the invalid time during imageforming, it becomes possible to operate the control electrode strips(row-control electrode strips 8R) at a fixed cycle of operation.Therefore, it is possible to simplify the control operation of each ofthe electrodes and the circuit configuration of the drivers, and hencereduce the cost of the apparatus.

What is claimed is:
 1. An image forming apparatus wherein an image isformed on a recording medium with the developer by performing voltageapplication in a time-divisional manner, to a plurality of controlelectrode strips having passage holes that allow the charged developerparticles to pass therethrough, characterized in that when voltageapplication is performed in the same direction as the arranged order ofthe control electrode strips with respect to the conveyed direction ofthe recording medium, the array pitch `d` of the control electrodestrips with respect to the conveyed direction of the recording medium isset so as to satisfy the following relation:

    d=(n+1/m)/x

(where n is a natural number) where x is the resolution of the image andm is the number of the control electrode strips.
 2. An image formingapparatus wherein an image is formed on a recording medium with thedeveloper by performing voltage application in a time-divisional manner,to a plurality of control electrode strips having passage holes thatallow the charged developer particles to pass therethrough,characterized in that when voltage application is performed in thereverse direction to the arranged order of the control electrode stripswith respect to the conveyed direction of the recording medium, thearray pitch `d` of the control electrode strips with respect to theconveyed direction of the recording medium is set so as to satisfy thefollowing relation:

    d=(n-1/m)/x

(where n is a natural number) where x is the resolution of the image andm is the number of the control electrode strips.